pio module for picoLisp.

This patch does a few other tiny things:

1. pio_mt_index() (originally from Lua)
   has been reused for all languages
   (including Lua) to serve as a generic
   helper function which returns the
   port/pin numeric identifier. This will
   avoid code duplication.

2. The same logic (point 1) for the func
   pio_decode()

3. re-structure PicoC's original pio code-
   now that all languages are equal-value-
   citizens. This had to be done in this
   commit because these functions are also
   applicable to picoLisp (Lua is an
   exception for now but we'll get to that
   later).

4. remove duplicate pio code in Lua section
   here: src/mocules/pio.